Unexpected House Pests

Yikes! There's something on crawling on your houseplants! In her first post of 2017, Jean Godawa tackles the subject of unexpected and unwelcome visitors that seem to appear out of nowhere.

Before the middle of the 17th century, people believed that some animals could spontaneously generate under the right conditions. According to the wisdom of the time, you could make a colony of mice simply by putting some dirty rags and a bit of wheat into a barrel. Likewise, if you left a carcass of meat hanging, you could create a swarm of flies. The idea that the right conditions just attract these creatures from somewhere else was not even considered. We know better now thanks to science and experimentation.

But if you've ever had a fittonia in your house develop mealy bugs, seemingly out of nowhere, you might start to understand that 17th century mid-set. If you keep your plants indoors, you shouldn't have to think about insects, should you?

Unfortunately, indoor plants are just as susceptible to certain pests as their outdoor counterparts, but not by spontaneous generation. Introducing new plants into your home, leaving windows open, putting houseplants outside during good weather are all invitations for unwanted pests.

A favourite meal to ladybugs, aphids (Aphididae) are not just pests of outdoor plants. These tiny (approx. 4 to 8 mm) bugs are easy to recognize with their pear shape and two short, stick-like cornicles near their backside. Aphids can have both winged and wingless forms and vary in colour depending on the species.

In their winged form, they can fly into your home and lay eggs on your houseplants. Aphids can also hide on fruits and vegetables at the grocery store where they have unlimited food and no predators. If you bring home any produce, even with a single aphid on it, your houseplants could soon be covered in the pests. Aphids are one of the few insects that are capable of parthenogenesis. That is, they can reproduce without mating.

These creatures use a short, beak-like mouth to pierce plant leaves or shoots and suck out the fluid. They excrete a clear, sticky substance called honeydew, which can often invite mold growth. Because of the way they feed, aphids are also capable of transmitting plant viruses from one plant to another.

Typically, a few aphids on a healthy houseplant won't kill it but they can become a problem if you don't intervene. If your houseplant is a sturdy one, take it outside and knock off the aphids with a forceful blast of water from the hose. For more delicate houseplants, treat aphids with an insecticidal soap or horticultural oil.

If your houseplants start to develop a cottony white substance on the underside of leaves or on the stems, take a closer look. That white mass could be a group of plant sucking mealybugs (Pseudococcidae). These creatures are elongated, oval bugs coated with a waxy secretion and are common pests of houseplants.

Mealybugs can be difficult to control because they spread easily. Being so small, they can crawl undetected from one plant to another or can be transferred by a slight breeze. The waxy coating protects the bugs from too much or too little moisture as well as from insecticidal sprays.

When introducing a new plant to the home, keep it quarantined, away from your other houseplants, for a couple of months to make sure it's healthy and pest free. For small infestations, pick the bugs off by hand or use a cotton swab dipped in rubbing alcohol to remove them. A small paintbrush dipped in mild dish soap can also help knock mealybugs off your plants. Unfortunately, you may need to discard a heavily infested houseplant to protect your other plants.

Another common houseplant and greenhouse pest is the whitefly (Aleyrodidae). At 2 to 3 mm long, this tiny pest can still cause quite a bit of damage. Like their aphid and mealybug cousins, whiteflies have a needle-like mouth they use to pierce plant tissue and suck out the fluid. Their feeding damages leaves and stems causing yellowing and wilting. The yellowish body and white wings of the whitefly are covered with a white powdery coating. These insects can also transmit plant viruses and can produce honeydew that fosters mold growth.

When immature whiteflies first hatch from their eggs, they crawl to a suitable area of the plant. As they develop, these larvae become oval, flattened and immobile until they pupate into winged adults. Insecticidal soaps are most effective on the immature bugs but will also work to get rid of the adults. Check the underside of leaves for the larvae if you notice whiteflies around your houseplants.

If leaves on your houseplants are dropping off or have yellow spotting (stippling) or webbing, particularly on the undersides, you may have a problem with spider mites (Acarina). These eight-legged creatures, relatives of spiders, may not be visible unless you look very closely, but their damage usually gives them away. Spider mites are another common destructive houseplant pest but they can be treated with insecticidal soap. As with all houseplant pests, it is better for the health of your other plants to discard a heavily infested one.

It would be easy to ignore or overlook scale insects (Coccoidea) as they look nothing like a typical insect. Throughout most of their immature lives, they have no antennae or legs and remain motionless while they feed. They often look like natural knots or bumps on stems. Some scale species are quite useful - shellac comes from a scale insect, as do some red dyes (cochineal). Despite this, many scale species are serious pests to indoor plants, and in large numbers can be very destructive.

While houseplants are typically protected from the multitude of potential pests outside, they are at a bit of a disadvantage indoors. An infested houseplant can't get help from any of the beneficial insects living in the garden. As much as I am fascinated by insects, I can't set a bunch of ladybugs loose in my home to deal with aphids and I'm not about to introduce parasitic wasps into my living room to get rid of scale insects on a ficus.

Our solutions for dealing with pests of indoor plants are more limited. Hand removal, trimming off infested leaves, applying insecticidal soap or horticultural oil can usually take care of most minor houseplant pests. 

The best option, though, is prevention. If you accept clippings from a friend or buy new plants, be sure to quarantine them first. Wash fruits and vegetables well when you bring them inside from the store or garden. If you put your plants outside in warm weather, frequently inspect them for unwanted pests.

If houseplant pests do show up, they could have hatched from eggs in the potting soil or entered through a slight crack around a window. Healthy plants can tolerate a few bugs and as long as you inspect plants often, and treat them early, pests are manageable.

Like mice in a barrel or rats on a ship, bugs don't spontaneously generate on indoor plants. Even though I know this, I am sometimes, very briefly, convinced that fruit flies can magically appear from another dimension. When I'm sure that I've thoroughly washed all the peaches in a basket from the market, those pesky little creatures will still be flying around my kitchen.

(Click this link to find out how to make your own insecticidal soap.)

Post written by Jean Godawa

Jean is a science teacher and writer. She has been writing science-related articles for print and online publications for more than ten years. Jean holds a degree in biology and environmental science with a focus on entomology from the University of Toronto. She has also conducted field research in the tropical rainforests of Asia and South America.

Photo Credits for this post: Many thanks to Leslie Ingram, David Capparet, Jeffery W. Lotz, Charles Olsen, Chazz Hesselein and Ken Sproule for allowing us to use of the photographs in this post.